Significant progress has been made in improving the critical current density (Jc) in various forms of MgB2 since superconductivity in this compound was discovered [1]. MgB2 exhibits the superconducting characteristics and physics of BCS-type LTS (low-temperature superconductor) materials, as evidenced for example by a significant isotope effect [2]; however, its critical temperature (Tc) is more than twice those of the presently used superconductors Nb3Sn and Nb3Al, and more than four times that of the present LTS workhorse, NbTi. The importance of MgB2 lies in its simple crystal structure, high critical temperature Tc, high critical current density (Jc), and large coherence length (hence transparency of grain boundaries to current flow). These properties of MgB2 offer the promise of important large-scale and electronic device applications. High Jc at a level of 105 A/cm2 to 106 A/cm2 at 20 K to 30 K for MgB2 wires have been reported by several groups [3-8]. However, Jc drops rapidly with increasing magnetic field. In all the studies on wires and bulk made from MgB2, Jc decreased more than 90% of its zero field value at 3 T within this temperature range due to the poor pinning ability of this material. If MgB2 is to be useful in high fields, the flux pinning strength must be improved. Attempts to enhance flux pinning have resulted in an encouraging improvement in irreversibility fields (Hirr) and Jc(H) performance in high fields by oxygen alloying in MgB2 thin films [9, 10] and by proton irradiation of MgB2 powder [11].
However, there has been some conjecture as to whether effective pinning centres can be introduced into MgB2 bulk or wires by a normal process such as chemical doping.
Several attempts have been made to improve flux pinning using chemical doping, but the results remain controversial. Furthermore, the results for doping into MgB2 are largely limited to addition rather than substitution. Zhao et al., have doped MgB2 with Ti and Zr, showing improvement of Jc in self field [12]. However, there is evidence for improved pinning as the Jc drops off rapidly with increasing field (Hirr=4T at 20 K). Recently, Wang et al doped MgB2 using nanoparticles [13]. The results showed an improvement of irreversibility field (Hirr) at 4.2 K for the doped sample. However, Hirr for the doped samples is not as good as the undoped ones at 20 K. Cimerle et al., found that doping with a small amount of Li, Al and Si showed some increase in Jc, but there is no improvement in Hirr [14].